1. Technical Field
The present invention is related to systems and methods for coating objects, and, more particularly, to systems and methods for building tamper resistant coatings on objects.
2. Background Art
There are many applications in which it is desired to apply a coating to a part or form a product layer using a spray process. Such applications may include applying primers, paints, and/or other types of coatings. Once such coating for which the present invention may be used, but is not limited to, is the application of a tamper resistant coating (TRC). A TRC is a layer applied to a product to provide a physical barrier to prevent inspection of, and tampering with, for example, the underlying circuitry and contents of electronic components.
Processes and systems for coating electronic circuits with protective coatings and security coatings using a thermal spray are generally known. Examples of such processes are described in U.S. Pat. Nos. 5,877,093; 6,110,537; 5,762,711; and 6,319,740 all to Heffner et al. Application of TRCs by heating a material to a molten state and spraying the molten material where desired is generally referred to herein as a “thermal-spray process,” a “thermal spray” or “thermal spraying.”
FIG. 1 illustrates a thermal-spray system disclosed by U.S. Pat. No. 6,110,537 to Heffner et al., which is incorporated herein by its reference. Particles of a coating material 60 are supplied from a feedstock supply to a thermal-spray gun 31. Fuel and oxygen are supplied to gun 31 to heat the coating material 60 to a molten state. Air 66 is combined with the stream of molten particles and output through flame front 67 toward one or more parts or circuits mounted on arms 68. During the thermal-spray process, arms 68 are rotated by motor 70. A coolant 74 may be pumped through the interior of arms 68 to regulate temperatures during the thermal-spray process.
Arms 68 rotate in the range of approximately one thousand revolutions per minute to repetitively sweep parts or circuits, e.g., attached at ends of arms 68, through the flame spray. With each pass, a layer of coating builds up on the exposed surface of an integrated circuit.
As shown in FIG. 2, U.S. Pat. No. 6,110,537 to Heffner et al. also discloses an embodiment wherein in lieu of arms 68, multiple integrated circuits 13 may be clamped in an aluminum disk fixture including disk 88 and mask 90 to a stand 92 having alignment pins 92a. Integrated circuits 13 are inserted and individually clamped in place by screws (not shown) that are inserted in holes 88b and 90b. Like arm 68 disk 88 contains internal coolant outlets 91 that connect with internal coolant passages 91a. The disk disclosed by Heffner et al. enables thermal-spray processing of multiple ICs, but requires a person to individually insert each circuit 13 in the disk before performing the thermal-spray process. Furthermore, each circuit must be individually removed from the disk by hand before further processing can be performed, if any is required.
The disk approach disclosed by Heffner et al. has subsequently evolved into the use of larger wheels and device-holding structures mounted thereto. With a larger wheel, a plurality of device-holding fixtures can be mounted and processed. Further, spray guns mounted on movable arms were incorporated to cover the increasing spray area on the larger spray-target wheels.
A shortcoming of such prior-art thermal-spray systems is that they do not produce a consistent TRC-deposition rate. An inconsistent TRC-deposition rate may unpredictably affect the porosity percentage and/or the total pore volume in a TRC-coating. This may make it difficult for any subsequent processing that must accurately account for porosity percentage and/or total pore volume. An inconsistent TRC-deposition rate may also unpredictably affect the deposited mass of a TRC-coating, which is problematic for mass-sensitive applications. Another problem that may result from an inconsistent TRC-deposition rate is that it may unpredictably affect the height of the completed TRC coating, and therefore, the height of the resulting integrated circuit package, for example, which is problematic for height-sensitive applications.
An inconsistent TRC-deposition rate may result from uncontrolled variation in any one of a number of different thermal-spray factors. One such factor is the temperature of the objects-under-process, such as integrated circuits. Thus, it is desirable to control the temperature of the objects during thermal spraying. This concern is not addressed by the prior art.
Instead, the following is generally done today. A number of practice runs are performed to establish an optimum set of thermal-spraying conditions, such as the torch settings, the speed to move the objects-under-process and the like. Then, thermal spraying is performed using the previously-determined optimum set of thermal-spraying conditions. The problem with this approach is that it does not take into account variations that may occur during thermal spraying from day to day, or even moment to moment. For example, as ambient temperature or a torch setting may drift over time, temperature of the objects-under-process may change. This may produce inconsistent TRC-deposition rates, and therefore, there is a need to control temperature of the objects-under-process.